This invention relates to inkjet printers and, more particularly, to techniques for increasing the life of a printhead having ink ejection elements and nozzles.
Inkjet printers are well known and widely used. These printers incorporate a scanning carriage which supports one or more inkjet print cartridges. Thermal inkjet print cartridges operate by rapidly heating a small volume of ink to cause the ink to vaporize and be ejected through one of a plurality of orifices or nozzles so as to print a dot of ink on a recording medium, such as a sheet of paper. Typically, the orifices are arranged in one or more linear arrays in a nozzle member. The properly sequenced ejection of ink from each orifice causes characters or other images to be printed on the paper as the printhead is moved relative to the paper. The paper is typically shifted each time the printhead is moved across the paper. The thermal inkjet printer is fast and quiet, as only the ink strikes the paper. One type of thermal printhead is described in U.S. Pat. No. 5,278,584, entitled xe2x80x9cInk Delivery System for an Inkjet Printhead,xe2x80x9d by Brian Keefe, et al., assigned to the present assignee and incorporated herein by reference.
A related type of inkjet printer uses piezo-electric elements, instead of heaters, to eject ink from an associated orifice. The present invention applies to both thermal and nonthermal inkjet printers.
One nonobvious drawback of such inkjet printers is that the ink ejection elements, whether heater resistors or piezoelectric elements, and their associated nozzles wear unevenly due to the nonuniform use of the various ink ejection elements and nozzles in the scanning printhead.
For example, in high-quality printing modes, only full lines of text are printed before the paper is shifted. A line of text will not be printed in two scans interrupted by a shifting of the paper. This is because the paper shifting mechanism would cause some noticeable misalignment between the dots forming characters printed in two consecutive scans interrupted by a shifting of the paper.
This full line printing mode is illustrated in FIG. 1, where a-single printhead 10 is shown making three scans across a sheet of paper, where the paper is shifted in the direction 12 after each scan. Three printed swaths are shown. Nozzles 14 in printhead 10 eject droplets of ink, and ink ejection elements below nozzles 14 are selectively fired to print full lines of text 16-20, shown as shaded bands. Although this example assumes a single scan before shifting the paper, the problem described would also occur if multiple scans were required for printing before the paper was shifted. A single scan may be in either direction across the paper.
After each scan, the paper is shifted so that the top of printhead 10 aligns with the first line of text 16, 18, 20 to be printed during the next scan.
If the bottom few nozzles 14 of printhead 10 cannot completely print a line of text during a single scan, then printing by the bottom nozzles is prevented or masked. The nozzles 14 below the top group of nozzles 14 frequently align with a space between the lines of text and so these middle nozzles 14, on average, are not used as frequently as the top nozzles 14. The third swath consisting of one line of text represents the end of a paragraph or the end of a page, so only the top nozzles 14 are used.
As seen, the nozzles and ink ejection elements at the top end of printhead 10 are used more frequently than the nozzles and ink ejection elements at the bottom and middle of printhead 10. This nonuniform use is reinforced many times per page since the top nozzles are always aligned with the first line to be printed in a scan.
The thermal, mechanical, chemical and frictional forces on the ink ejection elements and nozzles when ejecting ink cause nonuniform wear of these elements and nozzles. Such uneven wear effects both the drop volume and dot position accuracy and, thus, would require the inkjet print cartridge to be replaced sooner than if the ink ejection elements and nozzles had uniform wear. In extreme cases of such uneven wear, individual elements and nozzles may stop functioning entirely prior to the predicted replacement time.
Uneven wear also occurs due to the nature of type fonts because certain frequently used letters require more firing from some nozzles than from others during scanning of the printhead.
In certain recent printers (not necessarily prior art), a high resolution black ink printhead (e.g., one-half inch wide) is capable of printing a swath which is wider than one or more lower resolution color ink printheads (e.g., one-third inch wide) mounted in the same scanning carriage next to the black ink printhead. FIG. 2 shows such a black ink printhead 24 and a color ink printhead 26 scanning along direction 27.
When printing text or otherwise printing solely with black ink, the nozzles 28 and ink ejection elements in the black ink printhead 24 suffer from uneven wear as previously described. Additionally, when the black ink printhead 24 is used along with the color ink printhead 26 during color printing, only the shaded portions of printheads 24 and 26 are used, while the unshaded portion 30 of printhead 24 is not used. This, of course, results in uneven wear of the nozzles and ink ejection elements in the black ink printhead 24.
What is needed is a technique for printing which results in more uniform wear of the nozzles and ink ejection elements in inkjet printheads.
In one embodiment of the invention, an inkjet printer selectively energizes ink ejection elements in a scanning printhead such that the top nozzles of the printhead do not always print the first line of text in a scan. In one embodiment, this is accomplished by occasionally aligning the bottommost nozzle in the printhead with the bottom of the last full line of text to be printed during the scan. Also, nozzles which would normally print in the middle of the top line of text can be used to print the top rows of dots in the top line of text in a scan. This causes the nozzles which previously aligned with a space between text to be used more frequently to even out the nozzle usage.
The alignment of the printhead with respect to the lines of text to be printed is varied from page to page or varied at other intervals so that the top nozzles, middle nozzles, and bottom nozzles are used about equally.
The advancement of the paper through the print zone is controlled in conjunction with the selective control of the ink ejection elements to bring about the above-mentioned results.
The more uniform wearing of the nozzles and ink ejection elements extends the life of the printhead. This becomes more important with non-replaceable printheads which are not disposed of once a reservoir of ink is depleted.
In inkjet printers using a black ink printhead which has a wider printing swath than one or more color ink printheads in the same scanning carriage, a new printing technique makes use of the nozzles in the wider printhead which extend beyond the shorter printhead when both printheads are being used, such as for color printing. In this technique, a selected first pattern of ink ejection elements in the wider printhead extending beyond the shorter printhead is enabled during odd numbered firing cycles in a scan, while an opposite pattern of ink ejection elements at the other end of the wider printhead is enabled. During the even numbered firing cycles in a scan, the patterns are reversed. The paper is shifted after each scan such that the partial printing by the first pattern is overlapped by the partial printing by the opposite pattern in a subsequent scan. By overlapping these patterns, a full printing of dots results. The selective enabling (or disabling) of the ink ejection elements is performed by a print mask used in conjunction with the printing algorithm.
In another embodiment, the enabled patterns require more than two printing overlaps for full printing.